The present invention is directed to a multilayer optical information storage medium and more particularly to such a medium in which the information is reproduced through an incoherent signal generated through luminescence, fluorescence, electroluminescence, or the like. The present invention is further directed to a device for reading such a storage medium.
The best known currently used technology of mass optical storage is the CD-ROM. Information is stored in the form of pits in a spiral track on a medium. Coherent light is directed onto the track while the disk is spun to reproduce the information. A recent variation, the DVD-ROM, increases the recording density and also allows two layers per side.
However, the use of coherent light gives rise to diffraction effect and thus reduces the maximum possible recording density. Also, the number of layers per side is limited to two for the DVD-ROM and one for the CD-ROM. Further, the need to spin the disk increases the mechanical complexity of a device for reproduction of the information, especially since the current CD-ROM format requires a constant linear speed and thus a variable rotational speed.
It is an object of the invention to provide a storage medium that overcomes the above-noted deficiencies of the prior art.
To achieve this and other objects, the present invention is directed to a multilayer optical storage medium and to a method and apparatus for optically retrieving digital data stored as information pits in such a medium. The information is read without a need to spin the recorded medium, as it is necessary in the case of a known optical disc. The present invention offers advantages in terms of high information capacity, small size and relatively high axis speed.
The present invention is based on the reproduction of information in the form of an incoherent signal such as fluorescence, luminescence, or electroluminescence. Such an incoherent signal permits a higher spatial resolution in comparison to traditional coherent methods such as reflection, absorption and refraction (see in T. Wilson, C. Sheppard, xe2x80x9cTheory and Practice of Scanning Optical Microscopyxe2x80x9d, Academic Press, London, 1984). The use of the incoherent signal leads to as much as an eightfold increase in spatial resolution in a 3D optical memory.
The present invention has particular utility to a read-only device. In that embodiment, the data are stored in a multilayer structure having many optically thin information layers separated by isolating layers. Data bits are stored in information layers as pits containing a fluorescent material. Each layer of data is organized into a plurality of regions, called pages.
Various devices are taught for reproducing the information from each page. The medium and a detector are moved relatively in X, Y, and Z directions to access a desired page. The medium can be moved, or the medium can be held stationary, and the detector can be moved. The selected page is illuminated by a light source, which can be one of an array of light-emitting diodes. The light may pass through pages other than the desired page. However, only the image of the desired page is focused on the detector, and the unfocused images of the other pages can be removed through spatial filtering. Alternatively, various waveguide illumination techniques can be used to illuminate only a specific page.